Spatially Graded Millimeter Sized Mo1-xWxS2 Monolayer Alloys: Synthesis and Memory Effect

Controlled growth of 3R phase niobium diselenide and its properties

Inversion symmetry broken 3R phase transition metal dichalcogenides (TMDs) show fascinating prospects in spintronics, valleytronics, and nonlinear optics. However, the controlled synthesis of 3R phase TMDs is still a great challenge. In this work, two-dimensional 3R-NbSe2 single crystals up to 0.2 mm were synthesized for the first time through chemical vapor deposition method by designing a space-confined system. The crystal size and morphology can be controlled by the location of the stacked substrates and the amount of the Nb2O5 precursor. Scanning transmission electron microscopy and Raman measurements reveal the NbSe2 exhibits a pure 3R stacking mode with relatively weak interlayer van der Waals interactions. Importantly, 3R-NbSe2 shows obvious second harmonic generation signal which intensity intensified as thickness increases. Density functional theory calculations and optical absorption demonstrate the coexistence of metallic and semiconducting optical properties of 3R-NbSe2. We designed a NbSe2/WS2/NbSe2 photodetector utilizing the metallicity of 3R-NbSe2, which shows good performance especially an ultrafast response (6-7 μs, 0.5 ms - 7.9 s for Au electrodes in literature). The proposed strategy and findings are of great significance for the growth of many other 3R-TMDs and applications of nonlinear optical and ultrafast devices.

Air-stable self-powered photodetector based on TaSe2/WS2/TaSe2 asymmetric heterojunction with surface self-passivation

Two-dimensional (2D) transition metal dichalcogenides are highly suitable for constructing junction photodetectors because of their suspended bond-free surface and adjustable bandgap. Additional stable layers are often used to ensure the stability of photodetectors. Unfortunately, they often increase the complexity of preparation and cause performance degradation of devices. Considering the self-passivation behavior of TaSe2, we designed and fabricated a novel self-powered TaSe2/WS2/TaSe2 asymmetric heterojunction photodetector. The heterojunction photodetector shows excellent photoelectric performance and photovoltaic characteristics, achieving a high responsivity of 292 mA/W, an excellent specific detectivity of 2.43 × 1011 Jones, a considerable external quantum efficiency of 57 %, a large optical switching ratio of 2.6 × 105, a fast rise/decay time of 43/54 μs, a high open-circuit voltage of 0.23 V, and a short-circuit current of 2.28 nA under 633 nm laser irradiation at zero bias. Moreover, the device also shows a favorable optical response to 488 and 532 nm lasers. Notably, it exhibits excellent environmental long-term stability with the performance only decreasing ∼ 5.6 % after exposed to air for 3 months. This study provides a strategy for the development of air-stable self-powered photodetectors based on 2D materials.

Self-Driven Gr/WSe2/Gr Photodetector with High Performance Based on Asymmetric Schottky van der Waals Contacts

Two-dimensional (2D) self-driven photodetectors have a wide range of applications in wearable, imaging, and flexible electronics. However, the preparation of most self-powered photodetectors is still complex and time-consuming. Simultaneously, the constant work function of a metal, numerous defects, and a large Schottky barrier at the 2D/metal interface hinder the transmission and collection of optical carriers, which will suppress the optical responsivity of the device. This paper proposed a self-driven graphene/WSe2/graphene (Gr/WSe2/Gr) photodetector with asymmetric Schottky van der Waals (vdWs) contacts. The vdWs contacts are formed by transferring Gr as electrodes using the dry-transfer method, obviating the limitations of defects and Fermi-level pinning at the interface of electrodes made by conventional metal deposition methods to a great extent and resulting in superior dynamic response, which leads to a more efficient and faster collection of photogenerated carriers. This work also demonstrates that the significant surface potential difference of Gr electrodes is a crucial factor to ensure their superior performance. The self-driven Gr/WSe2/Gr photodetector exhibits an ultrahigh Ilight/Idark ratio of 106 with a responsivity value of 20.31 mA/W and an open-circuit voltage of 0.37 V at zero bias. The photodetector also has ultrafast response speeds of 42.9 and 56.0 μs. This paper provides a feasible way to develop self-driven optoelectronic devices with a simple structure and excellent performance.

TMDC ternary alloy-based triboelectric nanogenerators with giant photo-induced enhancement

Multifunctional self-powered energy harvesting devices have attracted significant attention for wearable, portable, IoT and healthcare devices. In this study, we report transition metal dichalcogenide (TMDC) ternary alloy (Mo0.5W0.5S2)-based self-powered photosensitive vertical triboelectric nanogenerator (TENG) devices, where the ternary alloy functions both as a triboelectric layer and as a photoabsorbing material. The scalable synthesis of the highly crystalline Mo0.5W0.5S2 ternary alloy can overcome the limitations of binary TMDCs (MoS2, WS2) by utilizing its superior optical characteristics, enabling this semiconductor-based TENG device to simultaneously exhibit photoelectric and triboelectric properties. Benefiting from visible light absorption, this vertical TENG device generates higher triboelectric outputs and exhibits excellent power harvesting properties under visible light illumination. The open circuit voltage and short circuit currents of the devices under illumination (410 nm, 525 μW cm-2) are enhanced by 62% and 253%, respectively, while in the darkness, a very high photoresponsivity of ∼45.5 V mW-1 (voltage mode) is exhibited, indicating the superior energy harvesting potential under ultralow illumination. Furthermore, the energy harvesting ability from regular human activities and the operation as artificial e-skin expands the multi-functionality of this TENG device, paving a pathway for simultaneous mechanical and photonic energy harvesting with self-powered sensing.

Photoluminescence, Reflection Contrast, Raman, and Second Harmonic Generation Spectroscopies Spatially Resolve Strain, Alloying, Defects, and Electronic Characteristics of Lateral MX2 Heterostructures

Lateral heterostructures of two-dimensional (2D) transition metal dichalcogenides offer promise as platforms for a wide variety of applications from exotic physics to environmental control. Further development and study of these heterostructures require characterization methods that assess the quality of the heterostructures. Here, we extend current characterization strategies to create photoluminescence (PL), Raman, reflection contrast, and second harmonic generation (SHG) maps of individual monolayer core-shell WS2-MoS2 lateral heterostructures that were synthesized via water vapor assisted chemical vapor transport. Together, these methods provide the correlations required to resolve the effects of excitons, trions, lattice defects, strain, and alloying. The comparisons show substantial differences, especially in the regions near and at the narrow heterointerface. Comparisons between the different spectral maps show the importance of metal alloying for understanding the electronic and spatial structures of heterostructures. The results are compared to previous work on similar lateral heterostructures created by different methods.

Controllable growth of wafer-scale monolayer transition metal dichalcogenides ternary alloys with tunable band gap

The tuning of band gap is very important for the application of two-dimensional (2D) materials in optoelectronic devices. Alloying of 2D transition metal dichalcogenides (TMDCs) is an important way to tune the wide band gap. In this study, we report a multi-step vapor deposition method to grow monolayer TMDC ternary alloy films with wafer scale, including Mo_1-xW_xS₂, Mo_1-xW_xSe₂and MoS_2xSe_2(1-x), which are accurately controllable in the elemental proportion (xis from 0 to 1). The band gap of the three 2D ternary alloy materials are continuously tuned for the whole range of metal and chalcogen compositions. The metal compositions are controlled by the as-deposited thickness. Raman, photoluminescence, elemental maps and TEM show the high spatial homogeneity in the compositions and optical properties across the whole wafer. The band gap can be continuously tuned from 1.86 to 1.99 eV for Mo_1-xW_xS₂, 1.56 to 1.65 eV for Mo_1-xW_xSe₂, 1.56 to 1.86 eV for MoS_2xSe_2(1-x). Electrical transport measurements indicate that Mo_1-xW_xS₂and MoS_2xSe_2(1-x)monolayers shown-type semiconductor behaviors, and the carrier types of Mo_1-xW_xSe₂can be tuned asn-type, bipolar andp-type. Moreover, this control process can be easily generalized to other 2D alloy films, even to quaternary or multi-element alloy materials. Our study presents a promising route for the preparation of large-scale homogeneous monolayer TMDC alloys and the application for future functional devices.

Bandgap Engineering in 2D Lateral Heterostructures of Transition Metal Dichalcogenides via Controlled Alloying

2D heterostructures made of transition metal dichalcogenides (TMD) have emerged as potential building blocks for new-generation 2D electronics due to their interesting physical properties at the interfaces. The bandgap, work function, and optical constants are composition dependent, and the spectrum of applications can be expanded by producing alloy-based heterostructures. Herein, the successful synthesis of monolayer and bilayer lateral heterostructures, based on ternary alloys of MoS_{2(1- x )} Se_{2 x} -WS_{2(1- x )} Se_{2 x} , is reported by modifying the ratio of the source precursors; the bandgaps of both materials in the heterostructure are continuously tuned in the entire range of chalcogen compositions. Raman and photoluminescence (PL) spatial maps show good intradomain composition homogeneity. Kelvin probe measurements in different heterostructures reveal composition-dependent band alignments, which can further be affected by unintentional electronic doping during the growth. The fabrication of sequential multijunction lateral heterostructures with three layers of thickness, composed of quaternary and ternary alloys, is also reported. These results greatly expand the available tools kit for optoelectronic applications in the 2D realm.

Layer-by-Layer Growth of AA-Stacking MoS2 for Tunable Broadband Phototransistors

The stacking configuration has been considered as an important additional degree of freedom to tune the physical property of layered materials, such as superconductivity and interlayer excitons. However, the facile growth of highly uniform stacking configuration is still a challenge. Herein, the AA-stacking MoS₂ domains with a ratio up to 99.5% has been grown by using the modified chemical vapor deposition through introducing NaCl molecules in the confined space. By tuning the growth time, MoS₂ domains would transit from an AA-stacking bilayer to an AAAAA-stacking five-layer. The epitaxial growth mechanism has been insightfully studied, revealing that the critical nucleation size of the AA-stacking bilayer is 5.0 ± 3.0 μm. Through investigation of the photoluminescence, the photoemission, especially the indirect photoexcitation, is dependent on both the stacking fashion and layer number. Furthermore, by studying the gate-tuned MoS₂ phototransistors, we found a significant dependence on the stacking configuration of MoS₂ of the photoexcitation and a different gate tunable photoresponse. The AAA-stacking trilayer MoS₂ phototransistor delivers a photoresponse of 978.14 A W^-1 at 550 nm. By correction of the external quantum efficiency with external field and illumination power density, it has been found that the photoresponse tunability is dependent on the layer number due to the strong photogating effect. This strategy provides a general avenue for the epitaxial growth of van der Waals film which will further facilitate the applications in a tunable photodetector.